(425e) Decarbonizing the Ammonia Industry Via Biomass-Based Chemical Looping Integrated with Ammonia Synthesis

The increasing global demand for ammonia, driven by population growth and agricultural needs, has underscored the urgency to decarbonize its production. The conventional ammonia synthesis process is primarily based on fossil fuels such as natural gas and coal, causing a tremendous strain on fossil fuel deposits and substantially increasing CO₂ emissions. The ammonia industry is one of the prominent contributors to global CO₂ emissions, with up to 500 million tons of CO₂ (≈1.8% of global emissions). Consequently, scientists and industrialists throughout the globe have shifted their attention towards sustainable feedstock as well as various CO₂ capture techniques. Amongst the alternative fuels, biomass stands out distinctively due to its tremendous availability, carbon-neutral nature, and high compatibility with the existing industrial infrastructure.

The conventional process for ammonia production involves a Steam Methane Reforming (SMR) process for the generation of raw gas (N₂ + H₂) followed by the Haber-Bosch process. In the SMR process, a two-reactor model is employed: the first reactor reforms natural gas using steam, followed by the second reactor, which reforms the unconverted natural gas using air, which also serves as a nitrogen source. The reformed natural gas, now syngas, further undergoes the Water Gas Shift (WGS) reaction followed by CO₂ removal using the Acid Gas Removal (AGR) process. The Haber-Bosch process requires a supply of high-purity H₂ and N₂ in a 3:1 ratio; hence, any further oxidizing agents are removed by the catalytic methanation process. The SMR process, coupled with AGR and post-combustion CO₂ capture, are highly energy-intensive processes requiring high fuel usage.

Chemical Looping (CL) is a fuel-flexible clean technology that utilizes varied feedstocks to produce high-purity hydrogen and sequestration-ready CO₂ by using redox reactions of the metal oxide particles. In the current process, a 3-reactor chemical looping system is developed with a carbon-neutral feedstock, i.e., biomass, to produce a high-purity mixture of H₂ and N₂ in a 3:1 ratio, which is directly sent to the Haber-Bosch process for ammonia synthesis. The biomass reacts with the oxygen carrier (OC) particles in the reducer reactor to produce a sequestration-ready CO₂ stream, simultaneously reducing the OC particles. The reduced OCs are then sent to the oxidizer reactor, where they are partially oxidized using a mixture of steam and depleted air. The partially oxidized OCs are further sent to the combustor reactor using a L-valve, wherein they react with the air and are completely oxidized to their original state. The air exiting the combustor reactor is nitrogen-rich, with oxygen compositions ranging from 2-5%. A fraction of this stream is sent to the oxidizer reactor such that the exiting gaseous mixture of H₂ and N₂ is in a 3:1 ratio. The OC particles used in the process are iron oxide based and have demonstrated excellent strength and recyclability with over 3000 cycles and <0.02% attrition rate.

The process is analyzed in ASPEN Plus software for its sensitivity toward the process parameters, and subsequently, an optimized process is developed. This process is then scaled up for an industrial output level and compared with the state-of-the-art SMR-based process. The chemical looping process is further demonstrated via experimental evaluation in a 2.5 kW_th moving bed reactor. The reducer reactor is designed for the middle injection of the fuel, ensuring counter-current reaction conditions are maintained for both char and volatiles. The results show 100% biomass conversion with 100% CO₂ dry purity. The reduced particles are then subjected to counter-current oxidation by steam and air in a similar moving bed reactor. Steam conversion as high as 60% and complete oxygen depletion is achieved yielding a pure H₂ + N₂ mixture. Both the reactors exhibit near thermodynamic conversions and hence prove the applicability of the process. A techno-economic analysis is performed for both ammonia production processes, and the results are compared using parameters such as the Levelized Cost of Product (LCOP) and Minimum Selling Price (MSP) of ammonia. The LCOP of both processes is found to be closer to 270 $/ton of NH₃, with the SMR process producing blue ammonia, whereas the CL process produces green ammonia. The MSP of products is calculated by considering respective credits, and a nearly 50% lower MSP can be achieved for the CL process than for the SMR process. Without considering the credits, a 2.5% lower MSP can be achieved using the CL process.

The key benefits demonstrated by the CL process include the use of carbon-neutral feedstock, inherent sequestration-ready CO₂ stream generation, autothermicity, long life of OCs, and lower cost of ammonia production. The CL process is autothermal in nature, meaning it does not require any external heat input, significantly lowering the energy requirements of the process. It not only generates renewable hydrogen but also inherently employs air separation, giving rise to an energy-efficient process for producing the reaction mixture for ammonia synthesis. The CL process effectively replaces the SMR reactors, WGS reactors, methanation reactor, AGR unit, and post-combustion CO₂ capture unit with a single chemical looping system, reducing the complexity of the process altogether. Furthermore, the pure stream of CO₂ generated in the reducer reactor can be utilized with ammonia to produce urea, an important compound in the fertilizer industry.